Studies of the activity of aortic smooth muscle in intact dogs will be continued. The work to date indicates that this activity is reflexly controlled. The functional significance of this reflex remains to be determined. Most of the research objectives as outlined in the original proposal are still valid. These objectives are as follows: 1) Determination of the receptor site and afferent pathways for this reflex. Indications are strong that these receptors are located in the left ventricle. The afferent pathways are definitely not located in the vagi but are perhpas located in cardiac sympathetic nerves. 2) Study of the effects of aortic smooth muscle responses on important cardiovascular parameters. These parameters include: aortic distensibility, pulse wave velocity, wave impedance and the compliance of the arterial system. 3) Investigation of the effects of aortic smooth muscle responses on aortic input impedance. By affecting cross- sectional area, distensibility and wave velocity, the aortic smooth muscle may control both the wave impedance and the input impedance in the aorta. In this way, aortic smooth muscle activity may affect the rate of dissipation of hydraulic energy in the arterial system. 4) Investigation of the functional significane of the smooth muscle responses. These studies can best be done, once the components of the reflex arc have been established so that the feed back loop may be opened. With the feed back look opened, the system may be studied in the absence of the aortic smooth muscle reflex. 5) A study of the distensibility distribution along the aorta. These experiments are nearing completion. 6) A study of the apparent differences in the viscoelastic behavior of the innervated, in situ aorta and the aorta after death. For all these studies on aortic smooth responses, methods have been developed to obtain aortic pressure-diameter relationship in intact anesthetized dogs. In these animals, aortic pressure, flow and diameter are recorded using catheter-tip instruments of adequate frequency response characteristics. These instruments do not require any surgical trauma to the aorta or its innervation.